1. Field of the Invention
Exemplary aspects of the present invention generally relate to a developing device employed in a copier, a printer, a facsimile machine, and so forth, and an image forming apparatus including the developing device.
2. Description of the Background
Related-art image forming apparatuses, such as a copier, a facsimile machine, a printer, or a multifunction printer having two or more of copying, printing, scanning, and facsimile functions, form a toner image on a recording medium (e.g., a sheet) according to image data using an electrophotographic method. In such a method, for example, a charger charges a surface of an image bearing member (e.g., a photoconductor); an irradiating device emits a light beam onto the charged surface of the photoconductor to form an electrostatic latent image on the photoconductor according to the image data; a developing device develops the electrostatic latent image with a developer (e.g., toner) to form a toner image on the photoconductor; a transfer device transfers the toner image formed on the photoconductor onto a sheet; and a fixing device applies heat and pressure to the sheet bearing the toner image to fix the toner image onto the sheet. The sheet bearing the fixed toner image is then discharged from the image forming apparatus.
Because it provides better durability and image forming performance, an image forming apparatus employing a developing device using a two-component developer including toner and a magnetic carrier is now widely used in electrophotography. The developing device using the two-component developer typically includes a supply path and a recovery path. The supply path supplies the developer to a developing sleeve serving as a developer bearing member while conveying the developer in a direction parallel to an axial direction of the developing sleeve. The recovery path returns the developer reaching a downstream end of the supply path to an upstream end thereof.
In some of the above-described developing devices including the supply path and the recovery path described above, the developer supplied from the supply path to the developing sleeve is returned to a downstream end of the supply path after development of a latent image, that is, after passing through a developing area where the developing sleeve and a photoconductor serving as a latent image bearing member contact each other. Some of the toner is supplied to the latent image formed on the photoconductor at the developing area, toner density declines. As a result, the toner density of the developer at the downstream end of the supply path relative to a direction of conveyance of the developer is decreased due to the developer having lower toner density being returned to the supply path, causing deterioration in image quality.
Published Unexamined Japanese Patent Application No. (hereinafter referred to as JP-A-) H05-333691 discloses a developing device including a supply path and a recovery path, disposed in parallel to a developing sleeve. In the developing device, developer supplied to the developing sleeve is conveyed to the recovery path after development, that is, after passing through a developing area. Specifically, the developer having lower toner density after passing through the developing area is not returned to the supply path but is instead conveyed to the recovery path. As a result, the toner density of the developer at a downstream end of the supply path relative to a direction of conveyance of the developer is not decreased.
However, in such a developing device, because the developer supplied from the supply path to the developing sleeve is not returned to the supply path, an amount of the developer at the downstream end of the supply path relative to the direction of conveyance of the developer is reduced. When unevenness in the amount of the developer occurs between the downstream and upstream ends of the supply path, the developer is unevenly supplied to the developing sleeve in an axial direction of the developing sleeve.
Specifically, the developer is evenly supplied to the developing sleeve at the upstream end of the supply path where a sufficient amount of the developer is present, providing stable image density. By contrast, the developer is not evenly supplied to the developing sleeve at the downstream end of the supply path where a smaller amount of the developer is present, causing uneven image density and irregular images.
To solve such problems, in the developing device disclosed in JP-A-H05-333691, a sufficiently larger amount of the developer is conveyed in the supply path compared with an amount of the developer supplied to the developing sleeve, making up for developer lost when supplied to the developing sleeve. As a result, unevenness in the amount of the developer in the supply path in the direction of conveyance of the developer can be reduced, preventing uneven image density.
However, in the above-described developing device, a driving speed of a developer conveyance member provided in the supply path is increased in order to increase conveyance speed of the developer so that the sufficiently larger amount of the developer is conveyed in the supply path. Consequently, larger stress is applied to the developer. Further, in a case in which a conveyance screw is used as the developer conveyance member, torque of the conveyance screw is increased when a rotation speed of the conveyance screw is increased in order to increase the conveyance speed of the developer in the supply path, and it is not possible to increase the amount of developer conveyed in the supply path sufficiently to prevent large stress to the developer and high torque of the conveyance screw.